The use of electronic devices such as mobile phones, notebook personal computers, and mobile game machines has spread in recent years, and lithium ion rechargeable batteries have been widely used as power sources for such devices.
When the battery voltage of a lithium ion rechargeable battery reaches a deep discharge region (for example, equal to or less than about 1.0 V), deposition of metallic lithium may cause an internal short-circuit. Charging the rechargeable battery in such a condition may lead to ignition or rupture of the rechargeable battery. Accordingly, to enhance safety, a battery pack has been equipped with a protection circuit in many cases in recent years, with the protection circuit having a function of prohibiting charging in the deep discharge state.
However, if the charge prohibition function is active, subsequent charging and discharging may no longer be performed (namely, the battery pack including the rechargeable battery is disabled). A state where the charge prohibition function is active may lead to complaints from users. Hence, to date, measures have been taken to prolong the time for a battery voltage to reach the deep discharge region, with such measures including a mode in which circuits in the battery pack are caused to sleep and a mode in which when the battery voltage becomes equal to or lower than a predetermined voltage, current consumed by the circuits in the battery pack is reduced in an over discharge region (Japanese Laid-open Patent Publication No. 2011-115012).
Such a related art reduces the current consumed after the voltage reaches the over discharge region and thus enables prolongation of the period for the voltage to reach the deep discharge region through the over discharge region and prolongation of a battery pack storable period.
The following documents are other examples of the related art: Japanese Laid-open Patent Publication No. 2010-124629; Japanese Laid-open Patent Publication No. 2000-32682; and Japanese Laid-open Patent Publication No. 2000-270485.